1. Field of the Invention
This invention relates to a high density silicon nitride sintered body having improved mechanical strength and a substantially crystallized intergranular phase and a method of producing the same, and more particularly to a high density silicon nitride body containing predetermined amounts of a sintering aid and at least one substance selected from Cr.sub.2 O.sub.3, MnO, CoO, NiO, Nb.sub.2 O.sub.5 and MoO.sub.3, and being capable of shaping into large-size products or mass-firing.
2. Related Art Statement
Silicon nitride sintered bodies are superior to metallic materials in mechanical stregth at high temperature, heat resistance, thermal shock resistance, corrosion resistance and the like, so that they are considered to be used for high temperature structural components, which can not be adapted in the metallic material, and the development of their applications is performed extensively.
Since silicon nitride can not easily be subjected to solid-phase sintering owing to the covalent bonding substance, it is subjected to liquid-phase sintering wherein additives such as MgO, SrO, Ce.sub.2 O.sub.3, Y.sub.2 O.sub.3, ZrO.sub.2, rare earth oxides, Al.sub.2 O.sub.3, AlN and the like are added to silicon nitride and a glassy phase is formed at a firing temperature to effect densification. Thus, the resulting sintered body contains a large amount of glassy phase produced during the firing at its grain boundary. Therefore, when such bodies are used in high temperature environments, the glassy phase in the grain boundaries is softened to degrade fatigue properties and oxidation resistance resulting from mechanical strength, creep deformation and creep rupture.
Consequently, many studies have been made with respect to a method for crystallizing the intergranular phase without the formation of a glassy phase. In Japanese Patent laid open No. 55-3,397, there is disclosed a method wherein silicon nitride is added with Y.sub.2 O.sub.3 and SiO.sub.2 and then fired to obtain a silicon nitride sintered body containing crystalline phases of Y.sub.2 O.sub.3.2SiO.sub.2 and 10Y.sub.2 O.sub.O.sub.3. 9SiO.sub.2. Si.sub.3 N.sub.4 in its grain boundary. In Japanese Patent laid open No. 56-59,674, there is disclosed a method wherein silicon nitride is added with Y.sub.2 O.sub.3 and then fired to obtain a silicon nitride sintered body containing a crystalline phase of xY.sub.2 O.sub.3. ySi.sub.3 N.sub.4 in its grain boundary. Further, in Japanese Patent laid open No. 59-8,670, a silicon nitride sintered body having an intergranular phase of melilite mineral facies represented by (Si,Mg,Y) (O,N) is also shown. Furthermore, in Japanese Patent Application Publication No. 58-50,994 , there is disclosed a method wherein a silicon nitride sintered body containing Y.sub.2 O.sub.3 or CeO.sub.2 is reheated to form a crystal of Y.sub.2 O.sub.3. Si.sub.3 N.sub.4 or Ce.sub.2 O.sub.3. Si.sub.3 N.sub.4 in the intergranular phase. These silicon nitride sintered bodies having crystallized intergranular phases all exhibit an improved high-temperature strength. In Japanese Patent Application No. 59-186,287, there is proposed a silicon nitride sintered body containing predetermined amounts of Y.sub.2 O.sub.3, MgO and CeO.sub.2, and having a crystallized intergranular phase obtained by sufficient densification through pressureless sintering and excellent high-temperature strength, oxidation resistance and static fatigue properties.
In these silicon nitride sintered bodies having the crystallized intergranular phase, the glassy phase in a grain boundary is crystallized at the cooling stage after densification through pressureless sintering or hot-press sintering. Japanese Patent laid open Nos. 59-207,879 and 59-213,676 disclose that when the silicon nitride sintered body containing rare earth elements and Group IIa elements as a sintering aid are sintered in a complicated or large size configuration, a secondary phase is ununiformly distributed and segregated in the intergranular phase to largely scatter the properties of the sintered body and reduce the strength, while the addition of a nitride or an oxynitride as a sintering aid enables the sintering in the complicated or large size configuration. Furthermore, in silicon nitride sintered bodies containing predetermined amounts of Y.sub.2 O.sub.3, MgO and CeO.sub.2, as shown in Japanese Patent Application No. 59-186,287, if the cooling rate is fast, the intergranular phase is not sufficiently crystallized, while if the cooling rate is slow, crystal grains in the intergranular phase largely grow to form coarse crystal grains in the intergranular phases. If the crystallization of intergranular phase is insufficient, the intergranular phase must be crystallized by a reheating treatment for improving high-temperature strength, oxidation resitance and static fatigue properties. On the other hand, if the crystal grains in the intergranular phase become coarser, cracks are produced at an interface between crystal grains in the intergranular phase to considerably reduce the strength. When the sintering aid is a nitride or an oxynitride, the composition of O, N in the intergranular phase varies and also the precipitated crystalline phase varies and consequently the properties degrade. Therefore, it is necessary to restrict the cooling rate in order to obtain the sintered body having a substantially crystallized intergranular phase composed of fine crystal grains.
In a silicon nitride sintered body having substantially crystallized intergranular phases as described above, if it is not intended to control the cooling stage after the firing, there results an unbalance between a crystal nucleus formation step and a crystal growth step constituting the crystallization stage and the crystallized intergranular phase is not favorably obtained. Therefore, it is necessary to limit the cooling rate. However, in mass-firing or firing into large size products, the sufficiently fast cooling rate is not obtained owing to heat capacity of the furnace or the product itself, and the crystal grains in the intergranular phase are coarsened to produce cracks in the interface between crystal grains, resulting in a remarkable reduction of the strength.